1. Field of the Invention
The invention relates to electrodialysis fluid treatment systems and is more particularly directed to electrode reversal in such systems.
2. Description of the Prior Art
Electrodialysis fluid treatment systems arranged in a stack configuration of a plurality of series connected, progressively purifying stages, are well known in the prior art. See, for example, U.S. Pat. No. 4,172,779, Yamaguchi, et al. Polarity reversal of the electrodes within the stages to prevent the buildup of scale is also well know. See, for example U.S. Pat. Nos. 2,694,680, Katz, et al., 2,863,813, Juda et al., and 3,341,441, Giuffrida et al., which fully illustrate the prior art and are hereby incorporated by reference.
In the present day operation of such stacks, electrode polarity reversal is an accepted technique which is effected up to several times per hour in a continuous cycle. As reversal is presently employed, all stages and all electrodes are changed at once. The character of the dilute product stream is also immediately changed upon reversal resulting in the contamination of partially desalted product in each stage. For instance, in a ten stage salt removal unit, the product contained in the final three or four stages is substantially desalted yet, upon reversal, the desalted or dilute stream now begins receiving the contaminated or salt concentrate material. As a result, the entire stack volume of product, whether partially desalted or not, must be discarded upon reversal with a consequential reduction in overall unit efficiency.
In contract to the prior art, it has now been discovered that the above described product loss may be avoided by sequentially changing the polarity of the electrodes stage by stage on a timed staggered basis. Reversal occurs in each electrical stage after the partially desalted product generated by that stage has worked its way through or cleared the hydraulic channels of the stage. Thus the great majority of product which has been partially desalted in the initial stages of the system will continue through the system without salt contamination and emerge in a completely usable state.